Key facts & rules

• Alpha (α): 2 protons + 2 neutrons (helium nucleus). Charge: +2. Stopped by paper or a few cm of air. Highly ionising.
• Beta (β): fast electron emitted from nucleus when neutron → proton + electron. Charge: −1. Stopped by a few mm of aluminium.
• Gamma (γ): high-frequency EM radiation. No charge, no mass. Stopped by thick lead or concrete.
• Half-life: time for activity (or number of undecayed nuclei) to halve.
• Background radiation: from rocks (radon), cosmic rays, food, medical sources.
• Nuclear fission: heavy nucleus splits into two smaller nuclei + neutrons + energy (used in power stations — uranium-235).
• Nuclear fusion: two light nuclei combine → heavier nucleus + energy. Requires extremely high temperatures (occurs in the Sun).
• Chain reaction: neutrons from fission trigger further fissions — controlled in reactors, uncontrolled in bombs.
• Notation: ᴬZX — A = mass number, Z = proton number (atomic number).